1. Field of the Invention
The present invention relates to a steering control device.
2. Description of the Related Art
There has hitherto been known an electric power steering device (hereinafter referred to as an “EPS”) that assists a driver in steering by applying power of a motor to a steering mechanism of a vehicle. Examples of the steering mechanism include a steering mechanism of a so-called rack-and-pinion type. Rotational motion of a steering shaft is converted into linear motion of a rack shaft through meshing between a pinion shaft and the rack shaft to change the steered angle of steered wheels. A control device for the EPS controls the motor on the basis of the results of detection performed by various sensors provided in the vehicle.
For example, a control device for an EPS described in Japanese Patent Application Publication No. 2015-42528 (JP 2015-42528 A) computes a basic assist component, which is a basic component of a steering assist force to be applied to a steering mechanism, on the basis of steering torque and the vehicle speed. In addition, the control device calculates a target pinion angle on the basis of basic drive torque, which is the total sum of the steering torque and the basic assist component, and computes a correction component for the basic assist component through feedback control in which the actual pinion angle is caused to coincide with the target pinion angle.
In order to increase a steering reaction force in accordance with an increase in lateral acceleration that acts on the vehicle, the control device computes a spring component of the basic drive torque on the basis of the target pinion angle, decreases the magnitude of the basic drive torque in accordance with the spring component, and computes a correction component for the basic assist component. The control device computes a first component of the spring component based on the target pinion angle and a second component of the spring component based on the lateral acceleration, and sets the proportions of use of the first component and the second component in accordance with the magnitude of the lateral acceleration. As the lateral acceleration becomes larger, the control device decreases the proportion of use of the first component, and increases the proportion of use of the second component.
The target pinion angle, and hence the correction component for the basic assist component, are decreased by an amount corresponding to a decrease in basic drive torque matching the spring component. As the correction component is decreased, the steering assist force to be applied to the steering mechanism is also decreased. The steering reaction force is increased in accordance with a decrease in steering assist force, and therefore a suitable steering reaction force can be obtained in accordance with the magnitude of the lateral acceleration. Thus, it is possible to harmonize the lateral acceleration to be received by the body of the driver and the steering reaction force (response) to be felt by hands of the driver through steering.
The first component of the spring component which is based on the target pinion angle contributes to a so-called sense of rigidity (sense of stability). The second component of the spring component, which is based on the lateral acceleration, contributes to a sense of integrity with the vehicle. Therefore, the sense of rigidity and the sense of integrity can be obtained suitably by adjusting the proportions of use of the first component and the second component in accordance with the lateral acceleration. For example, as the lateral acceleration is larger, it is conceivable that the sense of integrity with the vehicle is insufficient, and thus the proportion of use of the second component is increased. Consequently, it is possible to suitably obtain the sense of integrity with the vehicle in accordance with the magnitude of the lateral acceleration.
The control device according to JP 2015-42528 A, which sets the proportions of use of the first component of the spring component and the second component in accordance with the magnitude of the lateral acceleration, involves the following apprehensions. For example, when the vehicle is traveling on a low-friction road, the vehicle is less likely to be subjected to lateral acceleration. As the lateral acceleration is smaller, the proportion of use of the second component is decreased. Therefore, when the vehicle is traveling on a low-friction road, the spring component of the basic drive torque tends to be mainly composed of the first component. The first component of the spring component contributes to the so-called sense of rigidity, and therefore the driver feels a stronger sense of rigidity as a response. The driver may feel a sense of discomfort about obtaining a stable response on a low-friction road on which he/she is originally less likely to feel a stable response as a steering feel.